1. Field of The Invention
The present invention generally relates to an optical system for enlarging and projecting a small-scale image onto a screen. More particularly, the present invention relates to an optical system for adjusting a contrast ratio by controlling an iris in real time.
2. Description of The Related Art
Complying with the demand for larger screens and higher definition of an image display device, an image projection device is rapidly prevalent which enlarges and projects a small-scale image onto a large screen using an optical system. In particular, a rear projection device, which projects an image signal from a rear side of the screen, has come into wide use in a form of a projection television.
The general rear projection device includes a light source, a liquid crystal display (LCD) panel or a digital micromirror device (DMD) panel which is a display element for generating an image by use of light emitted from the light source, a projection optical system enlarging and projecting the image produced in the panel to a certain magnification, and a screen.
The image projection device focuses the light from the light source onto the DMD panel without yielding light loss, and projects the light illuminated from the DMD panel of the projection optical system with vividness and without distortion. A critical performance measure of the image projection device is a contrast ratio (C/R) representing the light intensity difference between the brightest white and the darkest black. A higher C/R represents a better image reproduction on the screen. The C/R is acquired using the following Equation.C/R=white luminance/black luminance  [Equation 1]
The conventional optical system cannot partially increase or decrease the amount of the light since its iris has a limited opening range (area). In addition, since the amount of light reflected from the surface of the digital light processing (DLP) projection device is received as it is, the C/R does not increase more than a certain value but is maintained with some constancy.
Even if the opening range of the iris is variable, the C/R increases only by adjusting the amount of light with respect to a specific image reproduction. The increase of the C/R requires the higher C/R of the projection optical system or the DMD panel, which is limited. Also, it is infeasible to continuously increase the C/R since the conventional iris cannot adjust the amount of light in association with the change in the image reproduction.
The conventional optical system has a disadvantage in that the amount of light is not effectively controlled since the position of the iris is restricted within the optical system.